From Egg to Tadpole: Early Morphogenesis in Xenopus
The development of a tadpole from a egg is described in great detail in most biology text books (e.g., Knox et al., 1994: p. 275; Purves et al., 1998: p. 883; Alberts et al., 1994; p. 1037) and numerous reviews. I will confine my brief comments to phenomena apparent in the video. For a very detailed description of morphogenesis, see Nieuwkoop & Faber (1967).
Early Cleavages and Symmetry of the Embryo. The sperm entry site becomes a key morphogenetic point in the egg, but after fertilization, the whole cortex of the cell is mobile and shifts about considerably. This mobility is particularly obvious in the video during gravity-induced cortical rotation immediately after eggs are turned upside down. Nieuwkoop & Faber (1967; p. 19) say that "...the first plane of cleavage coincides more or less with the dorso-ventral plane of bilateral symmetry" (their italics). However, Keller (1991; p. 65) says "... the first cleavage... does not define the future plane of bilateral symmetry" (his italics). According to Nieuwkoop & Faber, the first cleavage is not complete with final separation between the blastomeres brought about by a "... very thin partition wall in continuity with the ring-shaped cleavage furrow.". In the interior, these early divisions join up with complex partition walls invisible from the outside (Nieuwkoop & Faber, 1967). While initial cleavages are perpendicular to the surface, many cleavages from about the 7th. set onwards, are parallel to the surface creating two layers of cells, epithelial and non-epithelial. In addition, the pattern of cleavages soon gives rise to the blastocoel, an enlarging cell-free hollow within the embryo. Thus, the early cleavages appear deceptively simple in the video since they are seen only in surface view.
The first cleavage is symmetrical and the second slightly asymmetrical, creating two smaller dorsal and two larger ventral blastomeres. The third "equatorial" set of cleavages is highly asymmetrical, about a third down the egg from the animal pole. These cleavages create the quartets of micromeres and macromeres. My impression from many time-lapse sequences is that the cleavages from the third onwards are rarely executed with such precision and the pattern of cells generated at the early stages differs appreciably between embryos. This variability appears to have no consequences, and most embryos thereafter develop into normal tadpoles.
Later Cleavages. As is clearly visible in the video, early cleavages are synchronous, but they become increasingly asynchronous from about the 12th. cycle onwards. The progression of cleavages in the surface layer of cells is clearly evident from the changes in the surface layer of pigment particles.
Gastrulation. Wolpert is cited (Purves et al., 1998: p. 883) as saying that gastrulation is "...the most crucial event in our life, even surpassing birth and death in importance.."! Without gastrulation, none of the major tissues and organs of the body can differentiate.
Gastrulation occurs by an in-rolling and expansion of cell layers, initially those of the surface, in the process called involution. Coincidently, gastrulation creates a new gut cavity arching around the embryo, while the blastocoel shrinks. The events are complex and the opacity of the embryo makes it impossible to directly visualise the events in three dimensions. Thus, ideally the video images need to be accompanied in the classroom by diagrams showing the origin and movements of the various germ layers, notably the ectoderm, endoderm and mesoderm (e.g., see the diagrams in the text books above).
These movements are generated by the coordinated activity of thousands of individual cells, partly by pronounced changes in their shape which are visible in surface cells viewed at high magnification. Gastrulation begins on the surface with the elongation of bottle cells with constricted apices. The contraction of these cells initially creates a groove which rapidly deepens into the infolding that sweeps part of the surface into the interior of the embryo. The closing lip of the blastopore is called Spemann's Organiser (Organiser for short), after its discoverer; it seems to act as a center from which some subsequent events of embryogenesis are controlled. This end of the gut cavity becomes of anus of the digestive system.
Neurulation. Prior to neurulation, the very primitive notochord, a slender curving rod of differentiated cells, forms from the mesoderm just under the ectoderm (outer layer of cells) and elongates around the embryo. It gives rise to the backbone, both literally by differentiation, and from the evolutionary perspective. Formation of the notochord cannot be detected in these video sequences, but the next stage of differentiation is easy to follow. Just external to the notochord, the ectoderm thickens and then invaginates across the embryo to form a curved, sharp depression; one end terminates at the Organiser (above). Then this infolding seals tightly along its length to form the hollow cavity of the neural tube. Again these morphogenetic movements are generated by coordinated changes in the shape of cells on the surface of the embryo.
Morphogenesis in the spherical green alga Volvox offers an simpler, useful comparison with these embryos. The embryo of Volvox is a sphere consisting of a single layer of cells. Before reaching maturity, it turns itself completely inside out, a process known as inversion. During inversion, cells become longitudinal rigid while flexing within the layer is achieved by coordinated changes in shape due to contraction and movement of their surface. Both rigidity and the movements are generated by their cytoskeleton of microtubules and actin respectively and, as has long been recognised by biologists, inversion has a striking resemblance to neurulation. (For images of inversion and the shape changes of cells in living Volvox, see Pickett-Heaps & Pickett-Heaps, 1995.)
Further Development. Internally, embryo differentiation becomes exceedingly complex and discussion of all the details visible in the sequences (e.g., differentiation of the eye, gut, somites etc.) is beyond the scope of this guide.
Chromatophores. Cells responsible for generating color changes in animals are generically termed chromatophores. Fish scales are a valuable source of chromatophores since the cells can be isolated and grown on cover slips. They have an extensive system of radial microtubules over which the pigment particles move. In modelling pigment movement, particular importance is attributed to microtubule-motor proteins associated with the pigment (e.g., Haimo & Thaler, 1994) but a model based on motors alone is insufficient (Rodionov et al., 1998). Porter and colleagues (e.g., Porter and McNiven, 1982) suggest that the particles are held within a mesh that is moved in and out over the microtubules. Melanophores - chromatophores that contain black pigment - from Xenopus are also useful experimental model system (Rogers & Gelfand, 1999), sensitive to illumination (Daniolos et al., 1990). Living chromatophores and some simple experiments on them, are illustrated in Pickett-Heaps and Pickett-Heaps (1993).
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Knox, R.B., Ladiges, P.Y. & Evans, B. (1994). Biology. McGraw-Hill, Sydney; pp. 1067.
Nieuwkoop, P.D. & Faber, J. (1967). Normal Table of Xenopus laevis (Daudin). Hubrecht Lab., Utrecht; North-Holland Publishing Co., Amsterdam (2nd. edn.).
Pickett-Heaps, J.D. & Pickett-Heaps, J.F. (1993). "LIVING CELLS: Structure, Diversity and Evolution". 12" CAV videodisc; Sinauer Assoc., Sunderland, Mass., 1993.
Pickett-Heaps, J.D. & Pickett-Heaps, J.F (1995). "CELLULAR EVOLUTION in the Green Algae". 12" CAV videodisc; Cytographics, Ascot Vale, Australia.
Porter, K.R. & McNiven, M.A. (1982). The cytoplast: a unit structure in chromatophores. Cell 29: 23-32.
Purves, W.K., Orians, G.H., Heller, H.C. & Sadava, D. (1998). Life: the Science of Biology. Sinauer Assoc., Inc., Sunderland, Mass.; pp.1243.
Rodionov, V.I., Hope, A.J., Svitkina, T.M. & Borisy, G.G. (1998). Functional coordination of microtubule-based and actin-based motility in melanophores. Current Biology 8/#3: 165-168.
Rogers, S.L. & Gelfand, V.I. (1999). Myosin cooperates with microtubule motors during organelle transport in melanophores. Current Biology 8/#3: 161-164